In the formation of color photographic paper, a waterproof resin is melt extrusion coated on photographic grade cellulose paper. Polyethylene is commonly used and may be any one of a high density polyethylene, a low density polyethylene, a linear low density polyethylene, or a mixture of two or more of the above-mentioned polyethylenes.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene laminated to cellulose grade paper for use as a reflective receiver for thermal dye transfer imaging process. The preferred bonding agent in U.S. Pat. No. 5,244,861, to bond the biaxially oriented polypropylene sheets to paper, is low density polyethylene melt extruded from a slit die. Because of the high processing temperatures required for LDPE, shrinkage of the biaxially oriented sheet is common in the melt extrusion process. Shrinkage can cause undesirable changes in the Poisson ratio of the laminated receiver, as well as a reduction in the optical performance of the receiver. It would be desirable to reduce the extrusion temperature of the bonding layer and maintain acceptable integrity of the laminated support.
While the preferred bonding agent in U.S. Pat. No. 5,244,861 provides an acceptable bond between the biaxially oriented polypropylene sheet and cellulose paper at machine speeds of 300 meters/min., at machine speeds exceeding 400 meters/min., the bond strength falls below an acceptable level for product performance. It would be desirable to have a melt extruded bonding layer that would provide sufficient adhesion to both biaxially oriented sheets and imaging base paper at speeds greater than 400 meters/min.
In U.S. application Ser. No. 08/998,161 (Bourdelais et al.) filed Dec. 24, 1997, now U.S. Pat. No. 5,888,714, white pigment such as TiO.sub.2 added to the bonding layer has been proposed to improve photographic optical properties such as whiteness and opacity. In U.S. Pat. No. 5,476,708 (Reed et al.), it has been proposed that there exists an upper limit for pigment loading a polyolefin resin formulation because of manufacturing problems, such as melt extrusion die lines which significantly reduce the commercial value of imaging materials. It would be desirable to have a melt extruded bonding layer that could contain a high degree of TiO.sub.2 to improve optical properties without melt extruded die lines.
In U.S. Pat. No. 4,339,507 (Kurtz et al.), linear low density ethylene hydrocarbon copolymers for extrusion coating are proposed to improve adhesion. While linear low density polyethylene (LLDPE) does improve adhesion when compared to low density polyethylene (LDPE), LLDPE suffers from a phenomena referred to as draw resonance. Draw resonance is defined as the instability of the melt flow and is manifested as thickness variation both in the cross and machine directions. Draw resonance is a function of machine speed, becoming worse at higher line speeds. As a result, the maximum line speed at which an acceptable photographic paper can be manufactured is restricted by the onset of draw resonance.
During the manufacturing process for biaxially oriented laminated photographic papers discussed in U.S. application Ser. No. 08/998,161 (Bourdelais et al.) filed Dec. 24, 1997, the laminated structure is subjected to various forces during manufacturing that will cause delamination of the biaxially oriented polyolefin sheet from the paper. The delamination may be a result of bonding layer failure to either the base paper or the polypropylene sheet. Also, when the photographic paper is being processed and finished at photofinishers, the laminated structure is also subjected to various forces in both the wet and dry state. Furthermore, when the photographic paper is kept for years by the final customer, the laminated structure is subjected to forces created by temperature and humidity changes that could cause delamination of the biaxially oriented polyolefin sheets from the cellulose paper base. Delamination of the biaxially oriented sheet from the paper during manufacturing will result in the product being wasted, thus increasing the cost of manufacture. Delamination of the biaxially oriented sheet from the paper at either the photo finishing operation or in the final customer format will result in a loss in the appearance of the image and the reduction of the value of the photograph. It would be desirable if a melt extruded bonding adhesive could prevent delamination of biaxially oriented sheets from the base paper during manufacture of a laminated imaging support and in the final customer format.